A recombinant transforming murine retrovirus which contains the src gene of the avian retrovirus, Rous sarcoma virus, was utilized in a bone marrow colony-forming assay that detects transformation of murine hematopoietic cells. This virus was shown to be capable of transforming immature murine hematopoietic cells in vitro. Harvey and BALB murine sarcoma viruses were shown to promote the growth and differentiation of myeloid precursor cells in vitro. The induction of leukemogenesis by Moloney urine leukemia virus was demonstrated to be a multistep process. During the first stage, animals were viremic, but no overt symptoms appeared. In the second stage, the hematopoietic system underwent a greatly enhanced proliferative responses, while hematopoietic cells remained nonmalignant. Only the final stage of the disease was representative of an overtly malignant state in which spleen cells from the infected animal were capable of forming transformed colonies in soft agar and transplantable tumors in recipient animals. Several human hematopoietic tumors of diverse phenotypes were shown to possess transmissible onc genes by utilizing DNA transfection techniques. The onc genes of many were shown to be related to the neuroblastoma gene, n-ras, while one was related to the Kirsten murine sarcoma virus gene, kis. The activation of ras-related oncogenes appeared to be independent of hematopoietic tumor phenotype.